פירוש על קידושין 100:16

In both of these cases the first act does not cause the woman, or women to be betrothed because there was not a perutah’s worth given to each woman. When the man later sends gifts which are worth more than a perutah, we might have thought that these gifts can now act as the betrothal money. After all, he clearly intended to betroth her and she clearly agreed. However, the mishnah rules that since these gifts were not sent with the intent of effecting betrothal, they do not act as such. We also do not assume that the man realized that his first act of betrothal was invalid and that he is now sending betrothal money.

A minor cannot betroth a woman. Betrothal requires “awareness”, and minors legally lack the required “awareness.” Therefore, if he tries to betroth a certain woman, she is not betrothed. Even if he later on sends presents to the woman whom he tried to betroth, the presents do not effect kiddushin, for they were not sent as such but rather on account of the first act of betrothal, which was invalid.

The first clause of the mishnah contains two clauses. If the mishnah had taught only the case where he gave one perutah to two women, I might have thought that in that case, he thought that since he gave a perutah, the kiddushin was valid. Therefore, when he sent gifts later, he did not think of them as kiddushin. But in the case where he gave less than a perutah, we might have argued that everyone knows that one cannot perform kiddushin with less than a perutah. Therefore, when he sent presents, they were kiddushin. Thus the mishnah needed to teach us that even in this case, the presents are not considered kiddushin money.

In the cases in the first clause, I might have thought that the gifts were not kiddushin because the man might not have known that he had given less than a perutah for each woman he was betrothing. After all, it is hard to distinguish between a perutah and less than a perutah. He might have thought that he performed kiddushin, and that the gifts were just gifts.
But when it comes to the minor who betroths, we might have thought that everyone knows that minors cannot betroth. Therefore, when he sends gifts as an adult, this is kiddushin. Therefore, the mishnah had to teach us that also in this case the gifts do not count as kiddushin.

According to both of these amoraim, if a woman agreed to be betrothed and then the husband sent gifts, the gifts may be considered betrothal money.

The mishnah says that gifts are not considered betrothal money, which seems to contradict R. Huna and Rabbah. But Abaye explains this is only if there was an invalid act of betrothal that preceded the gifts. If there was no prior formal betrothal act, the gifts may actually be betrothal money, even though they were sent as gifts.

In this version, Rabbah uses the mishnah as a proof that gifts may be considered kiddushin. Only when they are sent because of a mistaken act are they not. This is similar to Abaye’s statement in the first version.

In this version, Abaye now rejects Rabbah. When the man has not performed any act of kiddushin, it is obvious that the gifts he sends are not kiddushin. But when he first betroths her I might have thought that the gifts are for the sake of kiddushin. That is why the mishnah needs to tells us that they are not.
So now we have a dispute between Abaye and Rabbah over whether gifts sent not following kiddushin can be concerned kiddushin.

R. Papa mediates between the two opinions. If the custom is to first betroth and then send gifts, the gifts might be a sign that betrothal occurred. But if the opposite, then the gifts are just gifts.

Even if a minority first send gifts and then betroth, we still need to be concerned lest the gifts are a sign that betrothal occurred.

This last section is about a case where a ketubah was seen by the people in the shuk—do we assume that this woman was married. Or perhaps the ketubah was written before betrothal?
R. Ashi explains that it depends on the custom. If they usually first betroth and then write a ketubah, then the ketubah might indeed be a sign of betrothal. But if they first write the ketubah and only later betroth, then it is not a sign that she was married.

This last section is about a case where a ketubah was seen by the people in the shuk—do we assume that this woman was married. Or perhaps the ketubah was written before betrothal?
R. Ashi explains that it depends on the custom. If they usually first betroth and then write a ketubah, then the ketubah might indeed be a sign of betrothal. But if they first write the ketubah and only later betroth, then it is not a sign that she was married.

A man cannot simultaneously marry a woman and her daughter or a woman and her sister. If he was already married to a certain woman and he attempted to betroth her daughter or sister, the betrothal would not be effective. The mishnah deals with a case where a man tried to betroth two such women simultaneously. Since they cannot both be effective neither is.

This is a classic rabbinic story, utterly packed with information. The main thing which we learn is that if one tries to simultaneously betroth two sisters, neither sister is betrothed. However, we also learn the following halakhot.
1) A man can betroth a group of women with one act of betrothal, and even if the betrothal is ineffective with some of the women (the sisters) it is effective with the others.
2) During the seventh year (the sabbatical year) a man can betroth using the agricultural produce of the women he is betrothing. This is because such produce is considered ownerless during the sabbatical year and when the man picks it up he owns it.
3) One woman can simultaneously accept kiddushin for herself and for other women.
By packing all of these details into one brief story, the story becomes an excellent didactic opportunity, far exceeding that which it is brought to explicitly demonstrate— that if one tries to simultaneously betroth two sisters, neither sister is betrothed

Rami b. Hama uses the verse that prohibits marrying two sisters to teach that if he marries them (or tries to) simultaneously, neither of them is considered married.

Rava points out that the verse prescribes karet (cutting off) for anyone who marries two sisters. But if the verse is talking about someone who tries to marry two sisters simultaneously and the betrothal is not valid, then why should he (or they) receive any punishment? He (and they) did not do anything.
Therefore, he explains that the mishnah follows Rabbah who said that when an act, such as marrying two sisters, cannot be performed consecutively, it can also not be performed simultaneously. If he tries to marry two sisters at the same time, neither are married.

If one gives 2/10 of one’s produce as tithe, the produce is considered tithed and may be eaten. But the tithes themselves are ruined since we do not know which part is the tithe and which part is not. But why, Abaye asks, is the second tenth considered even potentially a tithe? If I can’t first separate 1/10 and then another 1/10 both as tithes (indeed, one cannot), then when both are separated at the same time, neither of them should be tithes? This seems to refute Rabbah.